Network Working Group K. Moore
Request for Comments: 1894 University of Tennessee
Category: Standards Track G. Vaudreuil
Octel Network Services
January 1996
An Extensible Message Format for Delivery Status Notifications
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This memo defines a MIME content-type that may be used by a message
transfer agent (MTA) or electronic mail gateway to report the result
of an attempt to deliver a message to one or more recipients. This
content-type is intended as a machine-processable replacement for the
various types of delivery status notifications currently used in
Internet electronic mail.
Because many messages are sent between the Internet and other
messaging systems (such as X.400 or the so-called "LAN-based"
systems), the DSN protocol is designed to be useful in a multi-
protocol messaging environment. To this end, the protocol described
in this memo provides for the carriage of "foreign" addresses and
error codes, in addition to those normally used in Internet mail.
Additional attributes may also be defined to support "tunneling" of
foreign notifications through Internet mail.
Any questions, comments, and reports of defects or ambiguities in
this specification may be sent to the mailing list for the NOTARY
working group of the IETF, using the address
<notifications@cs.utk.edu>. Requests to subscribe to the mailing
list should be addressed to <notifications-request@cs.utk.edu>.
Implementors of this specification are encouraged to subscribe to the
mailing list, so that they will quickly be informed of any problems
which might hinder interoperability.
NOTE: This document is a Proposed Standard. If and when this
protocol is submitted for Draft Standard status, any normative text
(phrases containing SHOULD, SHOULD NOT, MUST, MUST NOT, or MAY) in
this document will be re-evaluated in light of implementation
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RFC 1894 Delivery Status Notifications January 1996
experience, and are thus subject to change.
1. Introduction
This memo defines a MIME [1] content-type for delivery status
notifications (DSNs). A DSN can be used to notify the sender of a
message of any of several conditions: failed delivery, delayed
delivery, successful delivery, or the gatewaying of a message into an
environment that may not support DSNs. The "message/delivery-status"
content-type defined herein is intended for use within the framework
of the "multipart/report" content type defined in [2].
This memo defines only the format of the notifications. An extension
to the Simple Message Transfer Protocol (SMTP) [3] to fully support
such notifications is the subject of a separate memo [4].
1.1 Purposes
The DSNs defined in this memo are expected to serve several purposes:
(a) Inform human beings of the status of message delivery processing, as
well as the reasons for any delivery problems or outright failures,
in a manner which is largely independent of human language;
(b) Allow mail user agents to keep track of the delivery status of
messages sent, by associating returned DSNs with earlier message
transmissions;
(c) Allow mailing list exploders to automatically maintain their
subscriber lists when delivery attempts repeatedly fail;
(d) Convey delivery and non-delivery notifications resulting from
attempts to deliver messages to "foreign" mail systems via a
gateway;
(e) Allow "foreign" notifications to be tunneled through a MIME-capable
message system and back into the original messaging system that
issued the original notification, or even to a third messaging
system;
(f) Allow language-independent, yet reasonably precise, indications of
the reason for the failure of a message to be delivered (once status
codes of sufficient precision are defined); and
(g) Provide sufficient information to remote MTA maintainers (via
"trouble tickets") so that they can understand the nature of
reported errors. This feature is used in the case that failure to
deliver a message is due to the malfunction of a remote MTA and the
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RFC 1894 Delivery Status Notifications January 1996
sender wants to report the problem to the remote MTA administrator.
1.2 Requirements
These purposes place the following constraints on the notification
protocol:
(a) It must be readable by humans as well as being machine-parsable.
(b) It must provide enough information to allow message senders (or the
user agents) to unambiguously associate a DSN with the message that
was sent and the original recipient address for which the DSN is
issued (if such information is available), even if the message was
forwarded to another recipient address.
(c) It must be able to preserve the reason for the success or failure of
a delivery attempt in a remote messaging system, using the
"language" (mailbox addresses and status codes) of that remote
system.
(d) It must also be able to describe the reason for the success or
failure of a delivery attempt, independent of any particular human
language or of the "language" of any particular mail system.
(e) It must preserve enough information to allow the maintainer of a
remote MTA to understand (and if possible, reproduce) the conditions
that caused a delivery failure at that MTA.
(f) For any notifications issued by foreign mail systems, which are
translated by a mail gateway to the DSN format, the DSN must
preserve the "type" of the foreign addresses and error codes, so
that these may be correctly interpreted by gateways.
A DSN contains a set of per-message fields which identify the message
and the transaction during which the message was submitted, along
with other fields that apply to all delivery attempts described by
the DSN. The DSN also includes a set of per-recipient fields to
convey the result of the attempt to deliver the message to each of
one or more recipients.
1.3 Terminology
A message may be transmitted through several message transfer agents
(MTAs) on its way to a recipient. For a variety of reasons,
recipient addresses may be rewritten during this process, so each MTA
may potentially see a different recipient address. Depending on the
purpose for which a DSN is used, different formats of a particular
recipient address will be needed.
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RFC 1894 Delivery Status Notifications January 1996
Several DSN fields are defined in terms of the view from a particular
MTA in the transmission. The MTAs are assigned the following names:
(a) Original MTA
The Original MTA is the one to which the message is submitted for
delivery by the sender of the message.
(b) Reporting MTA
For any DSN, the Reporting MTA is the one which is reporting the
results of delivery attempts described in the DSN.
If the delivery attempts described occurred in a "foreign" (non-
Internet) mail system, and the DSN was produced by translating the
foreign notice into DSN format, the Reporting MTA will still identify
the "foreign" MTA where the delivery attempts occurred.
(c) Received-From MTA
The Received-From MTA is the MTA from which the Reporting MTA
received the message, and accepted responsibility for delivery of the
message.
(d) Remote MTA
If an MTA determines that it must relay a message to one or more
recipients, but the message cannot be transferred to its "next hop"
MTA, or if the "next hop" MTA refuses to accept responsibility for
delivery of the message to one or more of its intended recipients,
the relaying MTA may need to issue a DSN on behalf of the recipients
for whom the message cannot be delivered. In this case the relaying
MTA is the Reporting MTA, and the "next hop" MTA is known as the
Remote MTA.
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Figure 1 illustrates the relationship between the various MTAs.
+-----+ +--------+ +---------+ +---------+ +------+
| | | | |Received-| | | | |
| | => |Original| => ... => | From | => |Reporting| ===> |Remote|
| user| | MTA | | MTA | | MTA | <No! | MTA |
|agent| +--------+ +---------+ +----v----+ +------+
| | |
| | <-------------------------------------------+
+-----+ (DSN returned to sender by Reporting MTA)
Figure 1. Original, Received-From, Reporting and Remote MTAs
Each of these MTAs may provide information which is useful in a DSN:
+ Ideally, the DSN will contain the address of each recipient as
originally specified to the Original MTA by the sender of the message.
This version of the address is needed (rather than a forwarding
address or some modified version of the original address) so that the
sender may compare the recipient address in the DSN with the address
in the sender's records (e.g. an address book for an individual, the
list of subscribers for a mailing list) and take appropriate action.
Similarly, the DSN might contain an "envelope identifier" that was
known to both the sender's user agent and the Original MTA at the time
of message submission, and which, if included in the DSN, can be used
by the sender to keep track of which messages were or were not
delivered.
+ If a message was (a) forwarded to a different address than that
specified by the sender, (b) gatewayed to a different mail system than
that used by the sender, or (c) subjected to address rewriting during
transmission, the "final" form of the recipient address (i.e. the one
seen by the Reporting MTA) will be different than the original
(sender-specified) recipient address. Just as the sender's user agent
(or the sender) prefers the original recipient address, so the "final"
address is needed when reporting a problem to the postmaster of the
site where message delivery failed, because only the final recipient
address will allow her to reproduce the conditions that caused the
failure.
+ A "failed" DSN should contain the most accurate explanation for the
delivery failure that is available. For ease of interpretation, this
information should be a format which is independent of the mail
transport system that issued the DSN. However, if a foreign error
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RFC 1894 Delivery Status Notifications January 1996
code is translated into some transport-independent format, some
information may be lost. It is therefore desirable to provide both a
transport-independent status code and a mechanism for reporting
transport-specific codes. Depending on the circumstances that
produced delivery failure, the transport-specific code might be
obtained from either the Reporting MTA or the Remote MTA.
Since different values for "recipient address" and "delivery status
code" are needed according to the circumstance in which a DSN will be
used, and since the MTA that issues the DSN cannot anticipate those
circumstances, the DSN format described here may contain both the
original and final forms of a recipient address, and both a
transport-independent and a transport-specific indication of delivery
status.
Extension fields may also be added by the Reporting MTA as needed to
provide additional information for use in a trouble ticket or to
preserve information for tunneling of foreign delivery reports
through Internet DSNs.
The Original, Reporting, and Remote MTAs may exist in very different
environments and use dissimilar transport protocols, MTA names,
address formats, and delivery status codes. DSNs therefore do not
assume any particular format for mailbox addresses, MTA names, or
transport-specific status codes. Instead, the various DSN fields
that carry such quantities consist of a "type" subfield followed by a
subfield whose contents are ordinary text characters, and the format
of which is indicated by the "type" subfield. This allows a DSN to
convey these quantities regardless of format.
2. Format of a Delivery Status Notification
A DSN is a MIME message with a top-level content-type of
multipart/report (defined in [2]). When a multipart/report content
is used to transmit a DSN:
(a) The report-type parameter of the multipart/report content is
"delivery-status".
(b) The first component of the multipart/report contains a human-
readable explanation of the DSN, as described in [2].
(c) The second component of the multipart/report is of content-type
message/delivery-status, described in section 2.1 of this document.
(d) If the original message or a portion of the message is to be
returned to the sender, it appears as the third component of the
multipart/report.
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NOTE: For delivery status notifications gatewayed from foreign
systems, the headers of the original message may not be available.
In this case the third component of the DSN may be omitted, or it
may contain "simulated" RFC 822 headers which contain equivalent
information. In particular, it is very desirable to preserve the
subject, date, and message-id (or equivalent) fields from the
original message.
The DSN MUST be addressed (in both the message header and the
transport envelope) to the return address from the transport envelope
which accompanied the original message for which the DSN was
generated. (For a message that arrived via SMTP, the envelope return
address appears in the MAIL FROM command.)
The From field of the message header of the DSN SHOULD contain the
address of a human who is responsible for maintaining the mail system
at the Reporting MTA site (e.g. Postmaster), so that a reply to the
DSN will reach that person. Exception: if a DSN is translated from a
foreign delivery report, and the gateway performing the translation
cannot determine the appropriate address, the From field of the DSN
MAY be the address of a human who is responsible for maintaining the
gateway.
The envelope sender address of the DSN SHOULD be chosen to ensure
that no delivery status reports will be issued in response to the DSN
itself, and MUST be chosen so that DSNs will not generate mail loops.
Whenever an SMTP transaction is used to send a DSN, the MAIL FROM
command MUST use a NULL return address, i.e. "MAIL FROM:<>".
A particular DSN describes the delivery status for exactly one
message. However, an MTA MAY report on the delivery status for
several recipients of the same message in a single DSN. Due to the
nature of the mail transport system (where responsibility for
delivery of a message to its recipients may be split among several
MTAs, and delivery to any particular recipient may be delayed),
multiple DSNs may be still be issued in response to a single message
submission.
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RFC 1894 Delivery Status Notifications January 19962.1 The message/delivery-status content-type
The message/delivery-status content-type is defined as follows:
MIME type name: message
MIME subtype name: delivery-status
Optional parameters: none
Encoding considerations: "7bit" encoding is sufficient and
MUST be used to maintain readability
when viewed by non-MIME mail
readers.
Security considerations: discussed in section 4 of this memo.
The message/delivery-status report type for use in the
multipart/report is "delivery-status".
The body of a message/delivery-status consists of one or more
"fields" formatted according to the ABNF of RFC 822 header "fields"
(see [6]). The per-message fields appear first, followed by a blank
line. Following the per-message fields are one or more groups of
per-recipient fields. Each group of per-recipient fields is preceded
by a blank line. Using the ABNF of RFC 822, the syntax of the
message/delivery-status content is as follows:
delivery-status-content =
per-message-fields 1*( CRLF per-recipient-fields )
The per-message fields are described in section 2.2. The per-
recipient fields are described in section 2.3.
2.1.1 General conventions for DSN fields
Since these fields are defined according to the rules of RFC 822, the
same conventions for continuation lines and comments apply.
Notification fields may be continued onto multiple lines by beginning
each additional line with a SPACE or HTAB. Text which appears in
parentheses is considered a comment and not part of the contents of
that notification field. Field names are case-insensitive, so the
names of notification fields may be spelled in any combination of
upper and lower case letters. Comments in DSN fields may use the
"encoded-word" construct defined in [7].
A number of DSN fields are defined to have a portion of a field body
of "xtext". "xtext" is used to allow encoding sequences of octets
which contain values outside the range [1-127 decimal] of traditional
ASCII characters, and also to allow comments to be inserted in the
data. Any octet may be encoded as "+" followed by two upper case
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RFC 1894 Delivery Status Notifications January 1996
hexadecimal digits. (The "+" character MUST be encoded as "+2B".)
With certain exceptions, octets that correspond to ASCII characters
may be represented as themselves. SPACE and HTAB characters are
ignored. Comments may be included by enclosing them in parenthesis.
Except within comments, encoded-words such as defined in [7] may NOT
be used in xtext.
"xtext" is formally defined as follows:
xtext = *( xchar / hexchar / linear-white-space / comment )
xchar = any ASCII CHAR between "!" (33) and "~" (126) inclusive,
except for "+", "\" and "(".
"hexchar"s are intended to encode octets that cannot be represented
as plain text, either because they are reserved, or because they are
non-printable. However, any octet value may be represented by a
"hexchar".
hexchar = ASCII "+" immediately followed by two upper case
hexadecimal digits
When encoding an octet sequence as xtext:
+ Any ASCII CHAR between "!" and "~" inclusive, except for "+", "\",
and "(", MAY be encoded as itself. (Some CHARs in this range may
also be encoded as "hexchar"s, at the implementor's discretion.)
+ ASCII CHARs that fall outside the range above must be encoded as
"hexchar".
+ Line breaks (CR LF SPACE) MAY be inserted as necessary to keep line
lengths from becoming excessive.
+ Comments MAY be added to clarify the meaning for human readers.
2.1.2 "*-type" subfields
Several DSN fields consist of a "-type" subfield, followed by a
semicolon, followed by "*text". For these fields, the keyword used
in the address-type, diagnostic-type, or MTA-name-type subfield
indicates the expected format of the address, status-code, or MTA-
name which follows.
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RFC 1894 Delivery Status Notifications January 1996
The "-type" subfields are defined as follows:
(a) An "address-type" specifies the format of a mailbox address. For
example, Internet mail addresses use the "rfc822" address-type.
address-type = atom
(b) A "diagnostic-type" specifies the format of a status code. For
example, when a DSN field contains a reply code reported via the
Simple Mail Transfer Protocol [3], the "smtp" diagnostic-type is
used.
diagnostic-type = atom
(c) An "MTA-name-type" specifies the format of an MTA name. For
example, for an SMTP server on an Internet host, the MTA name is the
domain name of that host, and the "dns" MTA-name-type is used.
mta-name-type = atom
Values for address-type, diagnostic-type, and MTA-name-type are
case-insensitive. Thus address-type values of "RFC822" and "rfc822"
are equivalent.
The Internet Assigned Numbers Authority (IANA) will maintain a
registry of address-types, diagnostic-types, and MTA-name-types,
along with descriptions of the meanings and acceptable values of
each, or a reference to a one or more specifications that provide
such descriptions. (The "rfc822" address-type, "smtp" diagnostic-
type, and "dns" MTA-name-type are defined in [4].) Registration
forms for address-type, diagnostic-type, and MTA-name-type appear in
section 8 of this document.
IANA will not accept registrations for any address-type, diagnostic-
type, or MTA-name-type name that begins with "X-". These type names
are reserved for experimental use.
2.1.3 Lexical tokens imported from RFC 822
The following lexical tokens, defined in [6], are used in the ABNF
grammar for DSNs: atom, CHAR, comment, CR, CRLF, DIGIT, LF, linear-
white-space, SPACE, text. The date-time lexical token is defined in
[8].
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RFC 1894 Delivery Status Notifications January 19962.2 Per-Message DSN Fields
Some fields of a DSN apply to all of the delivery attempts described
by that DSN. These fields may appear at most once in any DSN. These
fields are used to correlate the DSN with the original message
transaction and to provide additional information which may be useful
to gateways.
per-message-fields =
[ original-envelope-id-field CRLF ]
reporting-mta-field CRLF
[ dsn-gateway-field CRLF ]
[ received-from-mta-field CRLF ]
[ arrival-date-field CRLF ]
*( extension-field CRLF )
2.2.1 The Original-Envelope-Id field
The optional Original-Envelope-Id field contains an "envelope
identifier" which uniquely identifies the transaction during which
the message was submitted, and was either (a) specified by the sender
and supplied to the sender's MTA, or (b) generated by the sender's
MTA and made available to the sender when the message was submitted.
Its purpose is to allow the sender (or her user agent) to associate
the returned DSN with the specific transaction in which the message
was sent.
If such an envelope identifier was present in the envelope which
accompanied the message when it arrived at the Reporting MTA, it
SHOULD be supplied in the Original-Envelope-Id field of any DSNs
issued as a result of an attempt to deliver the message. Except when
a DSN is issued by the sender's MTA, an MTA MUST NOT supply this
field unless there is an envelope-identifier field in the envelope
which accompanied this message on its arrival at the Reporting MTA.
The Original-Envelope-Id field is defined as follows:
original-envelope-id-field =
"Original-Envelope-Id" ":" envelope-id
envelope-id = *text
There may be at most one Original-Envelope-Id field per DSN.
The envelope-id is CASE-SENSITIVE. The DSN MUST preserve the
original case and spelling of the envelope-id.
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RFC 1894 Delivery Status Notifications January 1996
NOTE: The Original-Envelope-Id is NOT the same as the Message-Id from
the message header. The Message-Id identifies the content of the
message, while the Original-Envelope-Id identifies the transaction in
which the message is sent.
2.2.2 The Reporting-MTA DSN field
reporting-mta-field =
"Reporting-MTA" ":" mta-name-type ";" mta-name
mta-name = *text
The Reporting-MTA field is defined as follows:
A DSN describes the results of attempts to deliver, relay, or gateway
a message to one or more recipients. In all cases, the Reporting-MTA
is the MTA which attempted to perform the delivery, relay, or gateway
operation described in the DSN. This field is required.
Note that if an SMTP client attempts to relay a message to an SMTP
server and receives an error reply to a RCPT command, the client is
responsible for generating the DSN, and the client's domain name will
appear in the Reporting-MTA field. (The server's domain name will
appear in the Remote-MTA field.)
Note that the Reporting-MTA is not necessarily the MTA which actually
issued the DSN. For example, if an attempt to deliver a message
outside of the Internet resulted in a nondelivery notification which
was gatewayed back into Internet mail, the Reporting-MTA field of the
resulting DSN would be that of the MTA that originally reported the
delivery failure, not that of the gateway which converted the foreign
notification into a DSN. See Figure 2.
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RFC 1894 Delivery Status Notifications January 1996
sender's environment recipient's environment
............................ ..........................................
: :
(1) : : (2)
+-----+ +--------+ +--------+ +---------+ +---------+ +------+
| | | | | | |Received-| | | | |
| |=>|Original|=>| |->| From |->|Reporting|-->|Remote|
| user| | MTA | | | | MTA | | MTA |<No| MTA |
|agent| +--------+ |Gateway | +---------+ +----v----+ +------+
| | | | |
| | <============| |<-------------------+
+-----+ | |(4) (3)
+--------+
: :
...........................: :.........................................
Figure 2. DSNs in the presence of gateways
(1) message is gatewayed into recipient's environment
(2) attempt to relay message fails
(3) reporting-mta (in recipient's environment) returns nondelivery
notification
(4) gateway translates foreign notification into a DSN
The mta-name portion of the Reporting-MTA field is formatted
according to the conventions indicated by the mta-name-type subfield.
If an MTA functions as a gateway between dissimilar mail environments
and thus is known by multiple names depending on the environment, the
mta-name subfield SHOULD contain the name used by the environment
from which the message was accepted by the Reporting-MTA.
Because the exact spelling of an MTA name may be significant in a
particular environment, MTA names are CASE-SENSITIVE.
2.2.3 The DSN-Gateway field
The DSN-Gateway field indicates the name of the gateway or MTA which
translated a foreign (non-Internet) delivery status notification into
this DSN. This field MUST appear in any DSN which was translated by
a gateway from a foreign system into DSN format, and MUST NOT appear
otherwise.
dsn-gateway-field = "DSN-Gateway" ":" mta-name-type ";" mta-name
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RFC 1894 Delivery Status Notifications January 1996
For gateways into Internet mail, the MTA-name-type will normally be
"smtp", and the mta-name will be the Internet domain name of the
gateway.
2.2.4 The Received-From-MTA DSN field
The optional Received-From-MTA field indicates the name of the MTA
from which the message was received.
received-from-mta-field =
"Received-From-MTA" ":" mta-name-type ";" mta-name
If the message was received from an Internet host via SMTP, the
contents of the mta-name subfield SHOULD be the Internet domain name
supplied in the HELO or EHLO command, and the network address used by
the SMTP client SHOULD be included as a comment enclosed in
parentheses. (In this case, the MTA-name-type will be "smtp".)
The mta-name portion of the Received-From-MTA field is formatted
according to the conventions indicated by the MTA-name-type subfield.
Since case is significant in some mail systems, the exact spelling,
including case, of the MTA name SHOULD be preserved.
2.2.5 The Arrival-Date DSN field
The optional Arrival-Date field indicates the date and time at which
the message arrived at the Reporting MTA. If the Last-Attempt-Date
field is also provided in a per-recipient field, this can be used to
determine the interval between when the message arrived at the
Reporting MTA and when the report was issued for that recipient.
arrival-date-field = "Arrival-Date" ":" date-time
The date and time are expressed in RFC 822 'date-time' format, as
modified by [8]. Numeric timezones ([+/-]HHMM format) MUST be used.
2.3 Per-Recipient DSN fields
A DSN contains information about attempts to deliver a message to one
or more recipients. The delivery information for any particular
recipient is contained in a group of contiguous per-recipient fields.
Each group of per-recipient fields is preceded by a blank line.
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RFC 1894 Delivery Status Notifications January 1996
The syntax for the group of per-recipient fields is as follows:
per-recipient-fields =
[ original-recipient-field CRLF ]
final-recipient-field CRLF
action-field CRLF
status-field CRLF
[ remote-mta-field CRLF ]
[ diagnostic-code-field CRLF ]
[ last-attempt-date-field CRLF ]
[ will-retry-until-field CRLF ]
*( extension-field CRLF )
2.3.1 Original-Recipient field
The Original-Recipient field indicates the original recipient address
as specified by the sender of the message for which the DSN is being
issued.
original-recipient-field =
"Original-Recipient" ":" address-type ";" generic-address
generic-address = *text
The address-type field indicates the type of the original recipient
address. If the message originated within the Internet, the
address-type field field will normally be "rfc822", and the address
will be according to the syntax specified in [6]. The value
"unknown" should be used if the Reporting MTA cannot determine the
type of the original recipient address from the message envelope.
This field is optional. It should be included only if the sender-
specified recipient address was present in the message envelope, such
as by the SMTP extensions defined in [4]. This address is the same
as that provided by the sender and can be used to automatically
correlate DSN reports and message transactions.
2.3.2 Final-Recipient field
The Final-Recipient field indicates the recipient for which this set
of per-recipient fields applies. This field MUST be present in each
set of per-recipient data.
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RFC 1894 Delivery Status Notifications January 1996
The syntax of the field is as follows:
final-recipient-field =
"Final-Recipient" ":" address-type ";" generic-address
The generic-address subfield of the Final-Recipient field MUST
contain the mailbox address of the recipient (from the transport
envelope) as it was when the message was accepted for delivery by the
Reporting MTA.
The Final-Recipient address may differ from the address originally
provided by the sender, because it may have been transformed during
forwarding and gatewaying into an totally unrecognizable mess.
However, in the absence of the optional Original-Recipient field, the
Final-Recipient field and any returned content may be the only
information available with which to correlate the DSN with a
particular message submission.
The address-type subfield indicates the type of address expected by
the reporting MTA in that context. Recipient addresses obtained via
SMTP will normally be of address-type "rfc822".
NOTE: The Reporting MTA is not expected to ensure that the address
actually conforms to the syntax conventions of the address-type.
Instead, it MUST report exactly the address received in the envelope,
unless that address contains characters such as CR or LF which may
not appear in a DSN field.
Since mailbox addresses (including those used in the Internet) may be
case sensitive, the case of alphabetic characters in the address MUST
be preserved.
2.3.3 Action field
The Action field indicates the action performed by the Reporting-MTA
as a result of its attempt to deliver the message to this recipient
address. This field MUST be present for each recipient named in the
DSN.
The syntax for the action-field is:
action-field = "Action" ":" action-value
action-value =
"failed" / "delayed" / "delivered" / "relayed" / "expanded"
Moore & Vaudreuil Standards Track [Page 16]

RFC 1894 Delivery Status Notifications January 1996
The action-value may be spelled in any combination of upper and lower
case characters.
"failed" indicates that the message could not be delivered to the
recipient. The Reporting MTA has abandoned any attempts to
deliver the message to this recipient. No further
notifications should be expected.
"delayed" indicates that the Reporting MTA has so far been unable to
deliver or relay the message, but it will continue to
attempt to do so. Additional notification messages may be
issued as the message is further delayed or successfully
delivered, or if delivery attempts are later abandoned.
"delivered" indicates that the message was successfully delivered to
the recipient address specified by the sender, which
includes "delivery" to a mailing list exploder. It does
not indicate that the message has been read. This is a
terminal state and no further DSN for this recipient should
be expected.
"relayed" indicates that the message has been relayed or gatewayed
into an environment that does not accept responsibility for
generating DSNs upon successful delivery. This action-
value SHOULD NOT be used unless the sender has requested
notification of successful delivery for this recipient.
"expanded" indicates that the message has been successfully delivered
to the recipient address as specified by the sender, and
forwarded by the Reporting-MTA beyond that destination to
multiple additional recipient addresses. An action-value
of "expanded" differs from "delivered" in that "expanded"
is not a terminal state. Further "failed" and/or "delayed"
notifications may be provided.
Using the terms "mailing list" and "alias" as defined in
[4], section 7.2.7: An action-value of "expanded" is only
to be used when the message is delivered to a multiple-
recipient "alias". An action-value of "expanded" SHOULD
NOT be used with a DSN issued on delivery of a message to a
"mailing list".
NOTE ON ACTION VS. STATUS CODES: Although the 'action' field might
seem to be redundant with the 'status' field, this is not the case.
In particular, a "temporary failure" ("4") status code could be used
with an action-value of either "delayed" or "failed". For example,
assume that an SMTP client repeatedly tries to relay a message to the
mail exchanger for a recipient, but fails because a query to a domain
Moore & Vaudreuil Standards Track [Page 17]

RFC 1894 Delivery Status Notifications January 1996
name server timed out. After a few hours, it might issue a "delayed"
DSN to inform the sender that the message had not yet been delivered.
After a few days, the MTA might abandon its attempt to deliver the
message and return a "failed" DSN. The status code (which would
begin with a "4" to indicate "temporary failure") would be the same
for both DSNs.
Another example for which the action and status codes may appear
contradictory: If an MTA or mail gateway cannot deliver a message
because doing so would entail conversions resulting in an
unacceptable loss of information, it would issue a DSN with the
'action' field of "failure" and a status code of 'XXX'. If the
message had instead been relayed, but with some loss of information,
it might generate a DSN with the same XXX status-code, but with an
action field of "relayed".
2.3.4 Status field
The per-recipient Status field contains a transport-independent
status code which indicates the delivery status of the message to
that recipient. This field MUST be present for each delivery attempt
which is described by a DSN.
The syntax of the status field is:
status-field = "Status" ":" status-code
status-code = DIGIT "." 1*3DIGIT "." 1*3DIGIT
; White-space characters and comments are NOT allowed within a
; status-code, though a comment enclosed in parentheses MAY follow
; the last numeric subfield of the status-code. Each numeric
; subfield within the status-code MUST be expressed without
; leading zero digits.
Status codes thus consist of three numerical fields separated by ".".
The first sub-field indicates whether the delivery attempt was
successful (2 = success, 4 = persistent temporary failure, 5 =
permanent failure). The second sub-field indicates the probable
source of any delivery anomalies, and the third sub-field denotes a
precise error condition, if known.
The initial set of status-codes is defined in [5].
Moore & Vaudreuil Standards Track [Page 18]

RFC 1894 Delivery Status Notifications January 19962.3.5 Remote-MTA field
The value associated with the Remote-MTA DSN field is a printable
ASCII representation of the name of the "remote" MTA that reported
delivery status to the "reporting" MTA.
remote-mta-field = "Remote-MTA" ":" mta-name-type ";" mta-name
NOTE: The Remote-MTA field preserves the "while talking to"
information that was provided in some pre-existing nondelivery
reports.
This field is optional. It MUST NOT be included if no remote MTA was
involved in the attempted delivery of the message to that recipient.
2.3.6 Diagnostic-Code field
For a "failed" or "delayed" recipient, the Diagnostic-Code DSN field
contains the actual diagnostic code issued by the mail transport.
Since such codes vary from one mail transport to another, the
diagnostic-type subfield is needed to specify which type of
diagnostic code is represented.
diagnostic-code-field =
"Diagnostic-Code" ":" diagnostic-type ";" *text
NOTE: The information in the Diagnostic-Code field may be somewhat
redundant with that from the Status field. The Status field is
needed so that any DSN, regardless of origin, may be understood by
any user agent or gateway that parses DSNs. Since the Status code
will sometimes be less precise than the actual transport diagnostic
code, the Diagnostic-Code field is provided to retain the latter
information. Such information may be useful in a trouble ticket sent
to the administrator of the Reporting MTA, or when tunneling foreign
nondelivery reports through DSNs.
If the Diagnostic Code was obtained from a Remote MTA during an
attempt to relay the message to that MTA, the Remote-MTA field should
be present. When interpreting a DSN, the presence of a Remote-MTA
field indicates that the Diagnostic Code was issued by the Remote
MTA. The absence of a Remote-MTA indicates that the Diagnostic Code
was issued by the Reporting MTA.
In addition to the Diagnostic-Code itself, additional textual
description of the diagnostic, MAY appear in a comment enclosed in
parentheses.
Moore & Vaudreuil Standards Track [Page 19]

RFC 1894 Delivery Status Notifications January 1996
This field is optional, because some mail systems supply no
additional information beyond that which is returned in the 'action'
and 'status' fields. However, this field SHOULD be included if
transport-specific diagnostic information is available.
2.3.7 Last-Attempt-Date field
The Last-Attempt-Date field gives the date and time of the last
attempt to relay, gateway, or deliver the message (whether successful
or unsuccessful) by the Reporting MTA. This is not necessarily the
same as the value of the Date field from the header of the message
used to transmit this delivery status notification: In cases where
the DSN was generated by a gateway, the Date field in the message
header contains the time the DSN was sent by the gateway and the DSN
Last-Attempt-Date field contains the time the last delivery attempt
occurred.
last-attempt-date-field = "Last-Attempt-Date" ":" date-time
This field is optional. It MUST NOT be included if the actual date
and time of the last delivery attempt are not available (which might
be the case if the DSN were being issued by a gateway).
The date and time are expressed in RFC 822 'date-time' format, as
modified by [8]. Numeric timezones ([+/-]HHMM format) MUST be used.
3.2.1.5 final-log-id field
The "final-log-id" field gives the final-log-id of the message that
was used by the final-mta. This can be useful as an index to the
final-mta's log entry for that delivery attempt.
final-log-id-field = "Final-Log-ID" ":" *text
This field is optional.
2.3.8 Will-Retry-Until field
For DSNs of type "delayed", the Will-Retry-Until field gives the date
after which the Reporting MTA expects to abandon all attempts to
deliver the message to that recipient. The Will-Retry-Until field is
optional for "delay" DSNs, and MUST NOT appear in other DSNs.
will-retry-until-field = "Will-Retry-Until" ":" date-time
The date and time are expressed in RFC 822 'date-time' format, as
modified by [8]. Numeric timezones ([+/-]HHMM format) MUST be used.
Moore & Vaudreuil Standards Track [Page 20]

RFC 1894 Delivery Status Notifications January 19962.4 Extension fields
Additional per-message or per-recipient DSN fields may be defined in
the future by later revisions or extensions to this specification.
Extension-field names beginning with "X-" will never be defined as
standard fields; such names are reserved for experimental use. DSN
field names NOT beginning with "X-" MUST be registered with the
Internet Assigned Numbers Authority (IANA) and published in an RFC.
Extension DSN fields may be defined for the following reasons:
(a) To allow additional information from foreign delivery status
reports to be tunneled through Internet DSNs. The names of such
DSN fields should begin with an indication of the foreign
environment name (e.g. X400-Physical-Forwarding-Address).
(b) To allow the transmission of diagnostic information which is
specific to a particular mail transport protocol. The names of
such DSN fields should begin with an indication of the mail
transport being used (e.g. SMTP-Remote-Recipient-Address). Such
fields should be used for diagnostic purposes only and not by
user agents or mail gateways.
(c) To allow transmission of diagnostic information which is specific
to a particular message transfer agent (MTA). The names of such
DSN fields should begin with an indication of the MTA
implementation which produced the DSN. (e.g. Foomail-Queue-ID).
MTA implementors are encouraged to provide adequate information, via
extension fields if necessary, to allow an MTA maintainer to
understand the nature of correctable delivery failures and how to fix
them. For example, if message delivery attempts are logged, the DSN
might include information which allows the MTA maintainer to easily
find the log entry for a failed delivery attempt.
If an MTA developer does not wish to register the meanings of such
extension fields, "X-" fields may be used for this purpose. To avoid
name collisions, the name of the MTA implementation should follow the
"X-", (e.g. "X-Foomail-Log-ID").
3. Conformance and Usage Requirements
An MTA or gateway conforms to this specification if it generates DSNs
according to the protocol defined in this memo. For MTAs and
gateways that do not support requests for positive delivery
notification (such as in [4]), it is sufficient that delivery failure
reports use this protocol.
Moore & Vaudreuil Standards Track [Page 21]

RFC 1894 Delivery Status Notifications January 1996
A minimal implementation of this specification need generate only the
Reporting-MTA per-message field, and the Final-Recipient, Action, and
Status fields for each attempt to deliver a message to a recipient
described by the DSN. Generation of the other fields, when
appropriate, is strongly recommended.
MTAs and gateways MUST NOT generate the Original-Recipient field of a
DSN unless the mail transfer protocol provides the address originally
specified by the sender at the time of submission. (Ordinary SMTP
does not make that guarantee, but the SMTP extension defined in [4]
permits such information to be carried in the envelope if it is
available.)
Each sender-specified recipient address SHOULD result in at most one
"delivered" or "failed" DSN for that recipient. If a positive DSN is
requested (e.g. one using NOTIFY=SUCCESS in SMTP) for a recipient
that is forwarded to multiple recipients of an "alias" (as defined in
[4], section 7.2.7), the forwarding MTA SHOULD normally issue a
"expanded" DSN for the originally-specified recipient and not
propagate the request for a DSN to the forwarding addresses.
Alternatively, the forwarding MTA MAY relay the request for a DSN to
exactly one of the forwarding addresses and not propagate the request
to the others.
By contrast, successful submission of a message to a mailing list
exploder is considered final delivery of the message. Upon delivery
of a message to a recipient address corresponding to a mailing list
exploder, the Reporting MTA SHOULD issue an appropriate DSN exactly
as if the recipient address were that of an ordinary mailbox.
NOTE: This is actually intended to make DSNs usable by mailing lists
themselves. Any message sent to a mailing list subscriber should
have its envelope return address pointing to the list maintainer [see
RFC 1123, section 5.3.7(E)]. Since DSNs are sent to the envelope
return address, all DSNs resulting from delivery to the recipients of
a mailing list will be sent to the list maintainer. The list
maintainer may elect to mechanically process DSNs upon receipt, and
thus automatically delete invalid addresses from the list. (See
section 7 of this memo.)
This specification places no restrictions on the processing of DSNs
received by user agents or distribution lists.
4. Security Considerations
The following security considerations apply when using DSNs:
Moore & Vaudreuil Standards Track [Page 22]

RFC 1894 Delivery Status Notifications January 19964.1 Forgery
DSNs may be forged as easily as ordinary Internet electronic mail.
User agents and automatic mail handling facilities (such as mail
distribution list exploders) that wish to make automatic use of DSNs
should take appropriate precautions to minimize the potential damage
from denial-of-service attacks.
Security threats related to forged DSNs include the sending of:
(a) A falsified delivery notification when the message is not delivered
to the indicated recipient,
(b) A falsified non-delivery notification when the message was in fact
delivered to the indicated recipient,
(c) A falsified Final-Recipient address,
(d) A falsified Remote-MTA identification,
(e) A falsified relay notification when the message is "dead ended".
(f) Unsolicited DSNs
4.2 Confidentiality
Another dimension of security is confidentiality. There may be cases
in which a message recipient is autoforwarding messages but does not
wish to divulge the address to which the messages are autoforwarded.
The desire for such confidentiality will probably be heightened as
"wireless mailboxes", such as pagers, become more widely used as
autoforward addresses.
MTA authors are encouraged to provide a mechanism which enables the
end user to preserve the confidentiality of a forwarding address.
Depending on the degree of confidentiality required, and the nature
of the environment to which a message were being forwarded, this
might be accomplished by one or more of:
(a) issuing a "relayed" DSN (if a positive DSN was requested) when a
message is forwarded to a confidential forwarding address, and
disabling requests for positive DSNs for the forwarded message,
(b) declaring the message to be delivered, issuing a "delivered" DSN,
re-sending the message to the confidential forwarding address, and
arranging for no DSNs to be issued for the re-sent message,
(c) omitting "Remote-*" or extension fields of a DSN whenever they would
otherwise contain confidential information (such as a confidential
forwarding address),
(d) for messages forwarded to a confidential address, setting the
envelope return address (e.g. SMTP MAIL FROM address) to the NULL
Moore & Vaudreuil Standards Track [Page 23]

RFC 1894 Delivery Status Notifications January 1996
reverse-path ("<>") (so that no DSNs would be sent from a downstream
MTA to the original sender),
(e) for messages forwarded to a confidential address, disabling delivery
notifications for the forwarded message (e.g. if the "next-hop" MTA
uses ESMTP and supports the DSN extension, by using the NOTIFY=NEVER
parameter to the RCPT command), or
(f) when forwarding mail to a confidential address, having the
forwarding MTA rewrite the envelope return address for the forwarded
message and attempt delivery of that message as if the forwarding
MTA were the originator. On its receipt of final delivery status,
the forwarding MTA would issue a DSN to the original sender.
In general, any optional DSN field may be omitted if the Reporting
MTA site determines that inclusion of the field would impose too
great a compromise of site confidentiality. The need for such
confidentiality must be balanced against the utility of the omitted
information in trouble reports and DSNs gatewayed to foreign
environments.
Implementors are cautioned that many existing MTAs will send
nondelivery notifications to a return address in the message header
(rather than to the one in the envelope), in violation of SMTP and
other protocols. If a message is forwarded through such an MTA, no
reasonable action on the part of the forwarding MTA will prevent the
downstream MTA from compromising the forwarding address. Likewise,
if the recipient's MTA automatically responds to messages based on a
request in the message header (such as the nonstandard, but widely
used, Return-Receipt-To extension header), it will also compromise
the forwarding address.
4.3 Non-Repudiation
Within the framework of today's internet mail, the DSNs defined in
this memo provide valuable information to the mail user; however,
even a "failed" DSN can not be relied upon as a guarantee that a
message was not received by the recipient. Even if DSNs are not
actively forged, conditions exist under which a message can be
delivered despite the fact that a failure DSN was issued.
Moore & Vaudreuil Standards Track [Page 24]

RFC 1894 Delivery Status Notifications January 1996
For example, a race condition in the SMTP protocol allows for the
duplication of messages if the connection is dropped following a
completed DATA command, but before a response is seen by the SMTP
client. This will cause the SMTP client to retransmit the message,
even though the SMTP server has already accepted it.[9] If one of
those delivery attempts succeeds and the other one fails, a "failed"
DSN could be issued even though the message actually reached the
recipient.
Moore & Vaudreuil Standards Track [Page 25]

RFC 1894 Delivery Status Notifications January 19966. Appendix - Guidelines for gatewaying DSNs
NOTE: This section provides non-binding recommendations for the
construction of mail gateways that wish to provide semi-transparent
delivery reports between the Internet and another electronic mail
system. Specific DSN gateway requirements for a particular pair of
mail systems may be defined by other documents.
6.1 Gatewaying from other mail systems to DSNs
A mail gateway may issue a DSN to convey the contents of a "foreign"
delivery or non-delivery notification over Internet mail. When there
are appropriate mappings from the foreign notification elements to
DSN fields, the information may be transmitted in those DSN fields.
Additional information (such as might be useful in a trouble ticket
or needed to tunnel the foreign notification through the Internet)
may be defined in extension DSN fields. (Such fields should be given
names that identify the foreign mail protocol, e.g. X400-* for X.400
NDN or DN protocol elements)
The gateway must attempt to supply reasonable values for the
Reporting-MTA, Final-Recipient, Action, and Status fields. These
will normally be obtained by translating the values from the remote
delivery or non-delivery notification into their Internet-style
equivalents. However, some loss of information is to be expected.
For example, the set of status-codes defined for DSNs may not be
adequate to fully convey the delivery diagnostic code from the
foreign system. The gateway should assign the most precise code
which describes the failure condition, falling back on "generic"
codes such as 2.0.0 (success), 4.0.0 (temporary failure), and 5.0.0
(permanent failure) when necessary. The actual foreign diagnostic
code should be retained in the Diagnostic-Code field (with an
appropriate diagnostic-type value) for use in trouble tickets or
tunneling.
The sender-specified recipient address, and the original envelope-id,
if present in the foreign transport envelope, should be preserved in
the Original-Recipient and Original-Envelope-ID fields.
The gateway should also attempt to preserve the "final" recipient
addresses and MTA names from the foreign system. Whenever possible,
foreign protocol elements should be encoded as meaningful printable
ASCII strings.
For DSNs produced from foreign delivery or nondelivery notifications,
the name of the gateway MUST appear in the DSN-Gateway field of the
DSN.
Moore & Vaudreuil Standards Track [Page 28]

RFC 1894 Delivery Status Notifications January 19966.2 Gatewaying from DSNs to other mail systems
It may be possible to gateway DSNs from the Internet into a foreign
mail system. The primary purpose of such gatewaying is to convey
delivery status information in a form that is usable by the
destination system. A secondary purpose is to allow "tunneling" of
DSNs through foreign mail systems, in case the DSN may be gatewayed
back into the Internet.
In general, the recipient of the DSN (i.e., the sender of the
original message) will want to know, for each recipient: the closest
available approximation to the original recipient address, the
delivery status (success, failure, or temporary failure), and for
failed deliveries, a diagnostic code that describes the reason for
the failure.
If possible, the gateway should attempt to preserve the Original-
Recipient address and Original-Envelope-ID (if present), in the
resulting foreign delivery status report.
When reporting delivery failures, if the diagnostic-type subfield of
the Diagnostic-Code field indicates that the original diagnostic code
is understood by the destination environment, the information from
the Diagnostic-Code field should be used. Failing that, the
information in the Status field should be mapped into the closest
available diagnostic code used in the destination environment.
If it is possible to tunnel a DSN through the destination
environment, the gateway specification may define a means of
preserving the DSN information in the delivery status reports used by
that environment.
Moore & Vaudreuil Standards Track [Page 29]

RFC 1894 Delivery Status Notifications January 19967. Appendix - Guidelines for use of DSNs by mailing list exploders
NOTE: This section pertains only to the use of DSNs by "mailing
lists" as defined in [4], section 7.2.7.
DSNs are designed to be used by mailing list exploders to allow them
to detect and automatically delete recipients for whom mail delivery
fails repeatedly.
When forwarding a message to list subscribers, the mailing list
exploder should always set the envelope return address (e.g. SMTP
MAIL FROM address) to point to a special address which is set up to
received nondelivery reports. A "smart" mailing list exploder can
therefore intercept such nondelivery reports, and if they are in the
DSN format, automatically examine them to determine for which
recipients a message delivery failed or was delayed.
The Original-Recipient field should be used if available, since it
should exactly match the subscriber address known to the list. If
the Original-Recipient field is not available, the recipient field
may resemble the list subscriber address. Often, however, the list
subscriber will have forwarded his mail to a different address, or
the address may be subject to some re-writing, so heuristics may be
required to successfully match an address from the recipient field.
Care is needed in this case to minimize the possibility of false
matches.
The reason for delivery failure can be obtained from the Status and
Action fields, and from the Diagnostic-Code field (if the status-type
is recognized). Reports for recipients with action values other than
"failed" can generally be ignored; in particular, subscribers should
not be removed from a list due to "delayed" reports.
In general, almost any failure status code (even a "permanent" one)
can result from a temporary condition. It is therefore recommended
that a list exploder not delete a subscriber based on any single
failure DSN (regardless of the status code), but only on the
persistence of delivery failure over a period of time.
However, some kinds of failures are less likely than others to have
been caused by temporary conditions, and some kinds of failures are
more likely to be noticed and corrected quickly than others. Once
more precise status codes are defined, it may be useful to
differentiate between the status codes when deciding whether to
delete a subscriber. For example, on a list with a high message
volume, it might be desirable to temporarily suspend delivery to a
recipient address which causes repeated "temporary" failures, rather
than simply deleting the recipient. The duration of the suspension
Moore & Vaudreuil Standards Track [Page 30]

RFC 1894 Delivery Status Notifications January 1996
might depend on the type of error. On the other hand, a "user
unknown" error which persisted for several days could be considered a
reliable indication that address were no longer valid.
8. Appendix - IANA registration forms for DSN types
The forms below are for use when registering a new address-type,
diagnostic-type, or MTA-name-type with the Internet Assigned Numbers
Authority (IANA). Each piece of information requested by a
registration form may be satisfied either by providing the
information on the form itself, or by including a reference to a
published, publicly available specification which includes the
necessary information. IANA MAY reject DSN type registrations
because of incomplete registration forms, imprecise specifications,
or inappropriate type names.
To register a DSN type, complete the applicable form below and send
it via Internet electronic mail to <IANA@IANA.ORG>.
8.1 IANA registration form for address-type
A registration for a DSN address-type MUST include the following
information:
(a) The proposed address-type name.
(b) The syntax for mailbox addresses of this type, specified using BNF,
regular expressions, ASN.1, or other non-ambiguous language.
(c) If addresses of this type are not composed entirely of graphic
characters from the US-ASCII repertoire, a specification for how
they are to be encoded as graphic US-ASCII characters in a DSN
Original-Recipient or Final-Recipient DSN field.
(d) [optional] A specification for how addresses of this type are to be
translated to and from Internet electronic mail addresses.
8.2 IANA registration form for diagnostic-type
A registration for a DSN address-type MUST include the following
information:
(a) The proposed diagnostic-type name.
(b) A description of the syntax to be used for expressing diagnostic
codes of this type as graphic characters from the US-ASCII
repertoire.
Moore & Vaudreuil Standards Track [Page 31]

RFC 1894 Delivery Status Notifications January 1996
(c) A list of valid diagnostic codes of this type and the meaning of
each code.
(d) [optional] A specification for mapping from diagnostic codes of this
type to DSN status codes (as defined in [5]).
8.3 IANA registration form for MTA-name-type
A registration for a DSN MTA-name-type must include the following
information:
(a) The proposed MTA-name-type name.
(b) A description of the syntax of MTA names of this type, using BNF,
regular expressions, ASN.1, or other non-ambiguous language.
(c) If MTA names of this type do not consist entirely of graphic
characters from the US-ASCII repertoire, a specification for how an
MTA name of this type should be expressed as a sequence of graphic
US-ASCII characters.
Moore & Vaudreuil Standards Track [Page 32]

RFC 1894 Delivery Status Notifications January 19969. Appendix - Examples
NOTE: These examples are provided as illustration only, and are not
considered part of the DSN protocol specification. If an example
conflicts with the protocol definition above, the example is wrong.
Likewise, the use of *-type subfield names or extension fields in
these examples is not to be construed as a definition for those type
names or extension fields.
These examples were manually translated from bounced messages using
whatever information was available.
Moore & Vaudreuil Standards Track [Page 33]

RFC 1894 Delivery Status Notifications January 19969.4 A delay report from a multiprotocol MTA. Note that there is no
returned content, so no third body part appears in the DSN.
From: <postmaster@nsfnet-relay.ac.uk>
Message-Id: <199407092338.TAA23293@CS.UTK.EDU>
Received: from nsfnet-relay.ac.uk by sun2.nsfnet-relay.ac.uk
id <g.12954-0@sun2.nsfnet-relay.ac.uk>;
Sun, 10 Jul 1994 00:36:51 +0100
To: owner-info-mime@cs.utk.edu
Date: Sun, 10 Jul 1994 00:36:51 +0100
Subject: WARNING: message delayed at "nsfnet-relay.ac.uk"
content-type: multipart/report; report-type=delivery-status;
boundary=foobar
--foobar
content-type: text/plain
The following message:
UA-ID: Reliable PC (...
Q-ID: sun2.nsf:77/msg.11820-0
has not been delivered to the intended recipient:
thomas@de-montfort.ac.uk
despite repeated delivery attempts over the past 24 hours.
The usual cause of this problem is that the remote system is
temporarily unavailable.
Delivery will continue to be attempted up to a total elapsed
time of 168 hours, ie 7 days.
You will be informed if delivery proves to be impossible
within this time.
Please quote the Q-ID in any queries regarding this mail.
--foobar
content-type: message/delivery-status
Reporting-MTA: dns; sun2.nsfnet-relay.ac.uk
Final-Recipient: rfc822;thomas@de-montfort.ac.uk
Status: 4.0.0 (unknown temporary failure)
Action: delayed
Moore & Vaudreuil Standards Track [Page 37]